A telescopic structure comprising an outer barrel and an inner barrel has heretofore been used widely in lens barrels for optical devices, such as cameras for taking motion/still pictures, for the purpose of performing focusing or zooming.
This type of telescopic structure is provided therein with focusing or zooming optical elements, and a movable barrel for adjusting them and a fixed barrel (at the side of a camera body, for example) are slidably coupled with each other in an inner and outer barrel relationship.
In conventional devices such as primitive telescopes, a structure has been used in which an inner barrel having an outer diameter and an outer barrel having an inner diameter substantially the same as the outer diameter of the inner barrel are simply slidably coupled with each other in one or more stages, and the positional relationship between respective barrels is manually adjusted so as to move straight. In modern optical devices, however, a modified structure is widely employed in which the relative positional relationship between respective barrels can be adjusted by a structure that converts a rotational movement of a focusing ring (or zooming ring) rotated manually or rotated by a drive means such as a motor to a relative movement amount of each barrel in the optical axis direction (thrust direction) using a screw mechanism such as helicoid or a spiral cam.
For small and light-weight lens having a short focal length, a structure is used in which an inner barrel and an outer barrel of a lens barrel are coupled so as to slide directly, making it possible to relatively easily provide accuracy in focusing or zooming operation and durability of sliding parts and mechanisms such as gears in the driving system. This also makes it possible for all of manual or motor driving systems to perform focusing or zooming adjustment with a small operation force within a reasonable range.
However, in a case where optical elements that are to be moved relatively and barrels for holding them are large and heavy such as in telephoto or super-telephoto lens having a focal length of, for example, 100 mm order, an operation force (e.g., motor driving torque) is required for relative movement of barrels when they have a coupling structure in which the inner barrel and the outer barrel directly slide in a simple manner. In addition, the friction due to contact of the barrels may possibly cause backlash and play, thereby deteriorating the accuracy and durability of the telescopic mechanism in itself or the driving mechanism such as gears and motors.
The technique disclosed in Patent Literature 1 below aims to solve the problems in terms of backlash and play of the barrels, accuracy, durability and operation force by employing a ball-bearing support between guide bars and lens holding frames that move relative to each other in the optical axis direction for a structure in which a mechanism is provided to restrict the relative movement of the barrels using a guide bar sliding mechanism.